Vehicle steering is generally controlled by a driver hand wheel that directs the angle of the vehicle road wheels used for steering. The movements of the driver hand wheel are transmitted to the vehicle road wheels by mechanical linkages and/or electronic components. The vehicle road wheels that change angle are generally located in the front of the vehicle in a system referred to as “front steering”. The angle of the road wheels is referred to as road wheel angle.
Active front steering (AFS) is a term referring to the use of electronic components to actively control or assist the steering of a vehicle so as to enhance steering performance beyond that possible by only direct mechanical linkages. There are many possible ways to enhance steering performance; for example, steering can be adapted to the weather conditions, to the behavior and habits of the driver, provide orderly stopping if the driver loses control, enhance the driver hand wheel control by changing steering characteristics, or provide driver control in the event of a steering mechanism malfunction.
At higher speeds large changes in the angle of the vehicle wheels can cause relatively large shifts in direction. Driver control at high speeds requires subtle changes in angle of the driver hand wheel. At low and medium speeds, a vehicle generally will be used for larger turns for parking or turning a corner. Large turns of the driver hand wheel are usually necessary to make large turns of the vehicle wheels. Driving is easier if the vehicle wheels turn less for driver hand wheel turns at high speed and more for driver hand wheel turns at low speed.
Variable gear ratio (VGR) steering is a method in the AFS system for adding and subtracting steering angle at the front wheels to the angle implied by the driver's hand wheel input. This can be accomplished by mechanical or electrical components. It is desirable to insure that the VGR system is fail-safe, does not vary greatly from its intended operational parameters, and operates in a safe manner. Lead steer is a method of anticipating the driver's intent at the hand wheel. VGR and lead steer may be combined to calculate the road wheel angle.
In an AFS system, the intended angle at the hand wheel and the actual angle at the front steering wheels are monitored to assure certain security metrics are met. In automotive parlance, a security metric is a safety performance requirement. An electronic AFS system uses actuators to rotate the front road wheels for a given road wheel angle. The actuators can have a three, or single phase motors to control the front wheel steering angles.
In an electronic AFS system the actuator motor is often controlled by a Pulse Width Modulation (PWM) signal. The PWM control is utilized to generate analog signals using the modulation of the pulse width or duty cycle of a periodic digital signal to produce controlled analog voltages. For example, if a 12 volt battery is connected to a device and the duty cycle is about 50% alternated between about 12 volts and about zero volts, the effective output voltage is about 50% of that of a constant 12 volts or six volts. Similarly, a duty cycle of X % may give an output voltage of X % of the voltage range and, accordingly, power available may be less than the total power. The quality and smoothness of the analog voltage output can be effectuated with capacitor, inductor, and resistor based passive component circuits. Various algorithms or (“control modes”) may use the PWM signal to control the behavior of a VGR system.
Precision control mode is used when the driver does not command significant changes in the target road wheel angle. When the commanded target road wheel angle is significantly small compared to the actual actuator angle, for example during high-speed highway driving, the AFS control system normally transitions to the precision control mode. Driver control at high speeds requires precision control of road wheel angle changes for subtle changes in angle of the driver hand wheel. The precision control uses only two phases of the actuator motor with a low PWM duty cycle to allow the driver to exit the precision control mode operation with large movements if the driver so desires.
Electric phase locking control mode is a method of electronically setting a fixed steering ratio in a VGR system. Electric phase locking also uses two phases of the actuator motor but at a high PWM duty cycle (for example, about 40%) to slow down the AFS actuator motor prior to mechanical locking of the AFS actuator when there are diagnosed errors in the system.
Proportional Integral Derivative (PID) control is a common method of control in industrial applications. The controller compares a measured output value from a process with a specified input value. The difference or “error” is used to calculate new process outputs to bring the process measured value back to the specified input value. A PID controller can adjust process inputs based on the history and the error signal rate of change yielding more stable and accurate control. A PID controller can also produce stable and accurate control when other algorithms would have a steady-state error or cause process oscillation. Cruise control in a car, a house thermostat, and electronic throttle control of vehicles and aircraft are common examples of how PID controllers are used to automatically adjust process inputs to hold a measured value to a specified reference value. In normal operation, the PID control keeps the AFS system's VGR actuator motor synchronized to a specified angle.
At present, an AFS system module with a low level control algorithm performs PID control, precision control, and electric phase locking control without differentiating between these cases. PID control is performed by controlling three phases of the AFS actuator motor. As mentioned above, precision control is performed by controlling only two phases of the AFS actuator motor with a low PWM duty cycle and the electric phase locking control is performed also by using two phases of the AFS actuator motor but at a high PWM duty cycle. The cases that need to be differentiated are transition from PID control or precision control to electric phase locking control, and transition from precision control to PID control. It is desirable to design a new system and method to allow an AFS system to securely transition from one control mode to another and to prevent unintentional transitions that may be undesirable, useless, or non-functional.